The invention relates to boats of the so-called planing or semiplaning type and has for an object to provide a hull design which in addition to permitting high maximum speed of the boat also causes relative little resistance at lower speeds when the boat is sailing more or less as a displacement boat.
Motor boats of today can be classified in two categories: Displacement boats and planing boats.
Displacement boats are running economically and performing nicely and conveniently at low speeds, but have a very limited maximum speed.
Planing boats are running easily at high speeds, but are in return both uneconomic and inconvenient at lower speeds, i.a. by causing big wash.
A boat buyer will very often desire a quick boat, if not for other reasons than for attaining fresh waters. When he has then come out there, it is of essential importance that the boat performs nicely at low speeds so that he can adapt the speed according to desire and conditions.
Today there does not exist any boat combining the desirable properties of both types of boats. It is true that there exist so-called semiplaning boats which in principle are displacements boats, but have a bottom design capable of affording a certain lift and hence permitting an increase of speed, but the speed is here still limited for reasons similar to those applying for normal displacement boats.
In order to attain high speeds with good utilization of the motor power, planing boats are normally designed with plane bottom faces and straight longitudinal lines, affording a lift so that the boat will slide on the water. Such boats are therefore mostly provided with a flat stern and made with a bottom width at the stern not much less than the maximum bottom width. Thus, researchers at the model tank of the Norwegian Technical University have found that the bottom width at the stern should not be less than 90% of the maximum width.
When such a boat is sailing with reduced speed as a displacement boat and hence lies relatively deeply in the water, the large stern gives rise to a big wake wave which apart from being a nuisance to the surroundings also means a considerable waste of power. When the boat is then to rise onto plane it will start by rearing, which requires considerable additional motor power.
If, on the other hand, the boat is given a more pronounced aft taper in order to behave better at low speed, the difficulties in rising onto plane will increase, due to suction by the water on the converging boat sides.
Considerable efforts have been devoted to reducing the water resistance during planing by ventilating the bottom by separate air supply and/or by shaping the bottom with more or less pronounced steps permitting the admission of air under the planing boat. The former expedient constitutes a complication causing increased costs, and the said steps in the bottom will only contribute in increasing the drawbacks referred to at low speed when the boat is sailing as a displacement boat.